Production of hot metal in a blast furnace requires a carbonaceous material such as coke, but reduction of the amount of consumption of the carbonaceous material per ton of hot metal (below, called the “reducing agent rate”) is a major objective for reducing manufacturing costs and has been pursued in the past.
For example, PLT 1 has the objective of reducing costs by increasing as much as possible the amount of consumption of small coal which was unable to be used in conventional blast furnace operations. PLT 1 discloses a method for operation of a blast furnace comprising blowing gas with an oxygen concentration of 40% or more from a tuyere at ordinary temperature, and the method comprises blowing the part of the pulverized coal containing +2 mm coarse grain coal in 5 to 30% and having a maximum grain size of 5 mm in the pulverized coal from the tuyere or near the tuyere into the furnace. Further, PLT 2 adjusts the ratio of the top gas blown from the blast furnace shaft tuyere and the top gas blown from the bottom tuyere to thereby match the amount of heat demand to the amount of heat supply of the blast furnace in the optimal state and enables remarkable improvement of the coke consumption and the charging efficiency over the known values. Further, PLT 3 discloses a method for operation of a blast furnace blowing fuel gas together with pulverized coal from a tuyere of the blast furnace to thereby secure the combustibility of the pulverized coal and improve the productivity and reduce the fuel cost (synonymous with reducing agent rate). Further, PLT 4 has the objective of stable, high productivity blast furnace operation and discloses a method for operation of a blast furnace blowing a gas with an oxygen concentration of 30% to less than 100% from the tuyere and blowing preheated gas from a shaft part at a middle stage of the blast furnace so thereby enable use of a large amount of pulverized coal.
Various technical innovations such as those explained above have enabled a remarkable improvement in the operating efficiency and led to the consumption of carbonaceous materials per ton of hot metal of a level below 500 kg.
In addition to such reduction of the reducing agent rate in blast furnace operation and other improvements of the manufacturing cost, in recent years, reduction of emissions of carbon dioxide (CO2), one of the hothouse gases mainly causing global warming, has become widely sought. The steel industry, one of the main industries related to CO2 emissions, has to respond to such social demands. Further reduction at blast furnace operations, which use large amounts of carbonaceous materials for iron and steel manufacture, is becoming urgent. The Japanese steel industry has established voluntary action targets to tackle reduction of CO2 emissions, but is being pressed to develop new technologies with eye to the future.
However, none of PLTs 1 to 4 has reduction of CO2 emissions as their main objectives. They do not sufficiently function to fundamentally cut the amount of generation of CO2. In this way, so long as based on existing operating methods, even if viewed in terms of heat efficiency, the situation now is that no further room can be found for major reduction in the carbon consumption.
In view of such a situation, work on developing technology aiming at a major reduction in the carbon consumption in a blast furnace operations has been proceeding in Europe. That is, in the so-called “ULCOS” project, a blast furnace process based on an oxygen blast furnace combining CO2 separation and recovery techniques, separating CO2 from the top gas, reheating it, and re-blowing it into the furnace from a tuyere newly provided at the side wall of the furnace body at the middle stage of the blast furnace or from the usual tuyere is being developed (NPLT 1).
FIG. 1 shows the flow of the above ULCOS blast furnace process. It is a process flow considered to be the highest for the effect of reduction of the carbon consumption of a blast furnace. The most different features from ordinary blast furnace operation are (1) the point of not using hot air for blast from the usual tuyere but blowing oxygen and pulverized coal at room temperature, (2) blowing top gas into the blast furnace after separating CO2 from all of the top gas to create “closed gas recycling”, and (3) heating recycled gas of the top gas to a high temperature at a time when blowing it from the usual tuyere. Further, in the flow of the blast furnace process of FIG. 1, the indirect reduction degree of ore is a high 89.7%. A 28% reduction ratio of carbon is achieved for the 289 kg/tHM of the amount of carbon (C) charged per ton of hot metal (1 tHM) at the time of normal operation. Further, the CO2 is separated from the top gas by the vacuum pressure swing adsorption method. The “vol” in FIG. 1 shows the amount of gas in the standard state.
These features pose serious risks when applied to commercial blast furnaces. That is, the above (1) requires that a large amount of pulverized coal is injected in so as to maintain the temperature of the combustion zone in front of the tuyere at a suitable value. According to a report of the ULCOS project, the pulverized coal rate (consumption of pulverized coal per ton of hot metal) has reached 300 kg/tHM and, as a result, the coke rate has fallen to 200 kg/tHM to less. With the current level of art of blast furnace operations which are only demonstrated at a generally 270 kg/tHM or more coke rate, it is not possible to easily create a stable operating state. In addition, since the oxygen is blown at room temperature, no sensible heat is input by the blown gas. Therefore, even if trouble occurs in operation causing the furnace to cool, the inside of the furnace cannot be quickly heated and it is difficult to restore operation. Further, the “closed gas recycling” operation of (2) has the risk of trace elements contained in the gas phase (for example, sulfur content etc.) being recycled and concentrating in the blast furnace process. There is a question as to whether stable operation can be maintained over a long period of time.
In this way, the blast furnace process aimed at by the ULCOS project would be hard to apply to a commercial blast furnace wherein hot metal production is required to be continued stably over a long period of time even if realization were possible on a short time test operation basis.
On the other hand, there is the method for cutting the carbon consumption by assigning hydrogen the reducing capacity that is one of the roles of carbon in blast furnace operation. That is, this is an operation blowing natural gas or coke oven gas (below, called “COG”) or other reducing gas containing hydrogen into the blast furnace. There are a large number of inventions relating to such an operation method, but in particular the method for modifying the mixed gas of CO2 and CO separated from the top gas to methane (CH4) and again blowing the modified gas to the blast furnace for the purpose of reducing CO2 emissions of blast furnaces has been disclosed (PLT 5).
This method separates and recovers CO2 (and/or CO) from the top gas, adds H2 to this to convert it to CH4, then again blows this into the blast furnace, but there are issues such as the new need for a CH4 conversion apparatus and the fact that if just blowing in CH4, carbon consumption of the blast furnace cannot be sufficiently reduced. It cannot be said that the social demand for reduction of CO2 emissions explained at the start can be sufficiently met.